In general, a switching power supply is extensively used for providing power supply. Most of the traditional power supply devices are linear power supply devices, which are usually used for an instrument providing a constant voltage and a constant current, which has a low ripple noise, a low EMI, a good modulation, and an easy-to-control features. Although the linear power supply device is popular, yet it still has certain shortcomings including a large power loss and a low power efficiency. Further, since the volume of the power supply device used for the instrument is large and inefficient, therefore it is a trend of using a switching-mode technique for the manufacture of the power supply device for instruments, and such technology is used to provide a power density and a power efficiency.
Please refer to FIG. 1 for the traditional switching power supply, which uses the repeated changes of electric connections and cutoffs of a circuit to switch a DC voltage to a specific frequency after the voltage is rectified and filtered. The result is filtered to obtain a fixed output voltage, instead of a voltage output not changing according to a programmable control.
Generally speaking, a switching power supply is a high frequency electronic device with an operating frequency ranging from 20 KHz to 200 KHz. In a system circuit, its power switch such as a MOSFET generally uses a transistor working in a saturation and cutoff area. However, the traditional linear power supply device generally uses a transistor working in a linear area and using it as a rheostat to modulate unstable input voltages. In this type of circuit, the passive component must bear a current that varies with its loading. Once there is a change to the input voltage or a sudden increase to the loading, then the power consumed by the passive component will change or increase accordingly. Therefore, the total system power loss will be increased, and the efficiency will drop. However, the switching power supply does not work completely in the linear area. Therefore, even the range of changes to the input voltage and the loading is very large, an efficiency higher than that of a linear power supply can be obtained.
Please refer to FIG. 2 for a flyback power supply circuit. Since its transformer also acts as an inductor for outputting stored energy, and the secondary terminal just needs a diode, and the C1 is mainly used for modulating the power factor of the power supply device.
Further, the power stage comprised of a pulse width modulation (PWM) IC, Q1, and T1 mainly uses the PWM IC to control the electric connection of the electronic switch of the transistor Q1. With the diode D1 and the capacitor CO of the secondary current, a DC voltage output is obtained. However, when the transistor Q1 is electrically connected, the primary current of the transformer T1 will have a primary current to pass through. Since the polarities of the primary and secondary currents of the transformer are opposite, the diode D1 will have a reverse bias voltage, and thus will not output any power, and is unable to have any feedback to the circuit to control the ON/OFF cycle of the PWM IC. Since the current is stored in the transformer T1, the transformer T1 will be worn out tremendously.
In view of the shortcomings of the above-mentioned traditional linear power supply device that once there is a change to the input voltage or a sudden increase to the loading, the power consumed by the passive component will change or increase accordingly, thus increasing the overall system power loss and lowering the efficiency accordingly, the inventor of the present invention based on years of experience on the manufacture and technological development of power supply devices to perform extensive researches, developments, and experiments, and finally invented a power supply device for outputting a stable power supply in accordance with the present invention.
In addition, although the primary terminal of the transformer is connected to a clamp circuit (comprised of D2 and D3) for clamping the voltage of the passing current, it still results in a lower efficiency than the original efficiency since the voltage of the primary current is higher. Furthermore, D2 and D3 must be a high voltage resisting diode, and its cost is higher than that of the general diodes.
Please refer to FIG. 3 for a forward power supply circuit. Its main difference from the flyback circuit resides on that the forward power supply circuit additionally uses a Schottky diode and an inductor at the secondary section of the power stage. When Q1 is cut off, the voltage polarity of the current isolated from the transformer is reversed, so that the voltage of the D2 diode becomes a reverse bias voltage and not electrically connected. However, the D3 diode is electrically connected. Then, the energy at the loading end is supplied by the energy stored in L0 and C0 via D3. Therefore, in the topology of the forward circuit, the L0 and C0 are also energy storage components besides acting as a low pass filter. The components adopted by the forward circuit are similar to those used by the flyback circuit, but since the reverse bias voltage from the Ti One Aspect Circuit is accumulated with the output voltage of C0 to double the output voltage when the Q2 transistor is turned off, therefore the voltage resistance of Q1 must be at least 800V. Although the forward circuit can reduce the primary and secondary current passing through the transformer which can reduce the copper loss of the transformer, the transformer adds a third coil and thus increases the cost.